Priority is claimed with respect to German application No. 197 44 691.4 filed in Germany on Oct. 10, 1997, the disclosure of which is incorporated herein by reference.
The invention is based on a method for determining a frequency of a signal, including converting an electrical signal present in the time domain to a corresponding digitized signal with use of an analog/digital converter; digital filtering the digitized signal for transforming the digitized signal to the frequency domain such that a line spectrum is generated which corresponds to the signal; and determining a signal frequency in the line spectrum from at least one predetermined spectral line belonging to the signal.
For many applications, particularly in the field of measuring technology as well as in electrical engineering, it is necessary to determine at least one frequency for a digitized signal that is present in the time domain, e.g. the frequencies of the underlying individual signals for a modulated signal. For this, the digitized signal is transformed, for example, to the frequency domain, e.g. by means of a complex Discrete Fourier Transformation (DFT) or a complex Fast-Fourier-Transformation (FFT) that is known per se. A line spectrum corresponding to the signal subsequently develops in the frequency domain, which signal can be evaluated in a known way and according to predetermined characteristics. For example, the carrier signal frequency can be determined for a modulated signal, e.g. by determining the position of at least one spectral line on the frequency axis and/or by measuring (frequency) intervals of predetermined spectral line. The accuracies possible with such methods are not sufficient for some application cases.
It is an object of the invention to improve a method of the type first described above in such a way that a predetermined, frequency measurement with very high accuracy is possible.
The above and other objects are accomplished in the context of the method first described above, wherein the method further includes:
repeating the digital filtering periodically in time, at least within one predetermined measuring interval, so that a temporally continuous sequence of transformation intervals develops within the measuring interval;
carrying out the digital filtering such that in each transformation interval at least one complex result vector (p0 to pn) is created, which has an amplitude component and a phase component (xcfx86);
selecting within the measuring interval, pairs of result vectors (p0, p1), which correspond to the same predetermined spectral line and belong to different, predetermined transformation intervals according to a predetermined pattern;
forming an associated phase difference (xcex94xcfx86) for each pair of selected result vectors (p0, p1);
determining a frequency F belonging to the selected result vectors from the phase differences (xcex94∠) in accordance with the following formula:   F  =            f      line        +          Δ      ⁢              xe2x80x83            ⁢                        F          line                ·                  1                      2            ⁢            π                          ·                                            ∑                              i                =                1                            n                        ⁢                          xe2x80x83                        ⁢                          Δ              ⁢                              xe2x80x83                            ⁢                              ϕ                i                                                                        ∑                              i                =                1                            n                        ⁢                          xe2x80x83                        ⁢                          Δ              ⁢                              xe2x80x83                            ⁢                              t                i                                                        
wherein
xcex94Fline=1/TFFT=1/(scanning rate for the analog/digital conversion xe2x80xa2 the number of scanning values within the transformation interval);
fline=the frequency of the line selected from the amplitude spectrum of a single transformation interval;
TFFT=length in time of the transformation interval;
xcex94ti=distance in time between selected pairs of result vectors.
A first advantage of the invention is that the method according to the invention for the most part is independent of intended and/or unintended interferences as well as environmental conditions, e.g. another superimposed signal that is nearly identical to the signal to be evaluated.
A second advantage is that the accuracy for determining the exact frequency can be specified, even for pulsed signals.
A third advantage is that the invention can also be used for high-frequency or highest frequency signals.
A fourth advantage is the high aging stability which can be achieved with an arrangement operated in accordance with this method. In addition, such an arrangement advantageously creates a high reproducibility of the measuring results since especially the so-called drifting is avoided.
Further features and advantages of the invention are revealed in the following detailed description.